Method for recovering metals from solutions

ABSTRACT

A method for recovering metals from solutions is described. A particular embodiment of the method concerns recovering copper metal from aqueous solutions containing copper ions, such as ammoniacal circuit-board etching solutions. A working embodiment of the invention comprises first treating a solution containing copper ions with a sufficient amount of an acid to obtain a solution pH of from about 1 to less than about 2.5. A reducing metal, such as iron, is then added to the solution to precipitate metal ions as metals. The reducing metal has a mesh size of about 230 or greater. The metal precipitate is recovered from the solution. The method is generally sufficiently efficient to allow discharge of solutions treated according to the method of the invention into public waste waters wherein the solution has less than about 5 ppm, and preferably less than 2 ppm, metal ions.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication, Ser. No. 08/193,484 (parent application), which was filedon Feb. 7, 1994, now U.S. Pat. No. 5,472,618. The parent application isincorporated herein by reference.

FIELD OF THE INVENTION

This invention concerns a method for recovering metals from solutions.

BACKGROUND OF THE INVENTION

The toxicity of metals has been well documented, and releasing effluentscontaining metals into the environment is now strictly controlled byenvironmental protection laws. For instance, the effluent limitations oncopper are as low as about 0.0017 pounds of copper per 1,000 pounds ofeffluent (about 1.7 parts per million). 40 C.F.R. § 421.52. Theelectronics industry generates large volumes of used circuit-boardetching solution containing high levels of copper ions. This effluentcannot be discharged directly into the environment without first beingsubjected to expensive procedures to reduce the effluent copperconcentration.

Some processes are known for removing metal ions from solution. Forinstance, Spedden et al.'s U.S. Pat. No. 3,634,071 concerns a processfor accelerating the precipitation of copper from solution. According toSpedden's process, a solution containing copper ions must be treatedwith sulfur dioxide. The purpose of the sulfur dioxide is to producehydronium ions (H⁺) by reacting with water to form sulfurous acid (H₂SO₃). This compensates for the consumption of hydronium ions that occursduring the reduction of copper ions. Spedden teaches that using sulfuricacid as the sole source of hydronium ions is unacceptable in a processfor recovering copper ions from solution.

Guess' U.S. Pat. No. 5,122,279 also describes a process for removingdissolved metals from water. The process comprises treating solutionscontaining heavy metals with ferrous dithionate to form a complex whichprecipitates from solution. Guess attempted a number of copper recoverytrials to compare his process to other processes for recovering copper.Example I of Guess discusses treating a feed solution with steel wool.Guess indicates that some removal of copper was visually evident as thesteel-wool mass gradually turned from steel color to copper-coloredfibers. Atomic absorption spectrometry of the treated solution indicatedthat only 28 percent of the copper in the solution was collected on thesteel wool.

Example VI of Guess discusses removing copper from solution using ironparticles. A liter of a solution having a pH of about 9 and containingcopper ions was treated with an equimolar quantity of iron particles. Asample of this solution was withdrawn for filtration after a coating ofmetallic copper was deposited on the surface of the iron particles.Analysis of the sample determined that approximately 87 percent of thecopper remained in solution. Hence, Example VI of the Guess patentteaches that only about 15 percent of the copper ions in solution can berecovered as copper metal by treating the solution with iron powders. A15 percent conversion is obviously unacceptable when the copperconcentration in an effluent can be no greater than about 2 ppm.

Guess also discusses U.K. Patent Application GB 125828 A. Thisapplication describes a process involving contacting a solutioncontaining copper ions with steel wool. According to Guess, the problemsassociated with this method include: (1) an uneconomically lowconversion of copper ion to copper metal; (2) a high cost associatedwith steel wool; and (3) a high labor cost associated with handling thematerials.

Despite ongoing investigations, a process for efficiently andinexpensively recovering metals from solutions containing ions of themetal still has not been developed. Although it is known to treatsolutions with iron to precipitate low levels of copper, this reactionhas not been effectively tailored for recovering copper ions from usedcircuit-board etching solutions. Moreover, the patents discussed aboveteach that simply using iron to recover copper metal is an inefficientand unacceptable method for recovering copper from solutions containingcopper ions.

SUMMARY OF THE INVENTION

The present invention provides a process for recovering metals fromaqueous solutions. The method comprises treating aqueous solutionscontaining ions of a first metal, such as copper (II), with afinely-divided reducing metal within a controlled pH range toprecipitate the first metal from solution. The invention addresses theproblems identified in the Background of the Invention for prior relatedprocesses. A preferred process according to the present inventionaccomplishes at least two goals: (1) The metal ions in solution arereduced to the zero oxidation state to produce reusable metals. Forinstance, copper (II) is reduced to copper (0). The recovered coppermetal preferably should have a sufficient purity to allow for subsequentuse, such as greater than about 60 percent pure. (2) The concentrationof copper (II) ions remaining in solution is reduced to be withinapplicable environmental regulations, such as from about 2 ppm to about3 ppm, prior to disposal or discharge into the environment.

One embodiment of the method comprises treating an aqueous solutioncontaining metal ions with a sufficient amount of an inorganic acid toobtain a pH of less than about 2.5. Thereafter, a finely-divided metalreducing agent is added to the solution in an amount sufficient toreduce substantially completely the dissolved metal ions to metals(i.e., a metal in the zero oxidation state). The reducing metal added tothe solution has an oxidation potential greater than the metal ions insolution. Without limitation, a preferred inorganic acid is sulfuricacid, and a preferred reducing metal is iron.

The size of the metal particles also appears to influence the efficiencyof the reaction. When iron is used as the reducing agent, the ironpreferably should have a mesh size of 230 or greater (larger mesh sizecorrelates with smaller particles). The iron powder may be added to thesolution batchwise or continuously, and the solution may be agitatedduring the addition.

A particular embodiment of the invention comprises treating a solutioncontaining copper ions with a sufficient amount of an inorganic acid toobtain a pH of less than about 2.5. A finely-divided metal powder isthen added to the solution in an amount sufficient to substantiallycompletely reduce the copper ions to copper metal. The metal preferablyhas a mesh size of 230 or greater and an oxidation potential greaterthan copper. The copper metal is then recovered, such as by filtration,with the solution thereafter having a copper ion concentration of lessthan about 5 ppm, preferably from about 2 ppm to about 3 ppm, and evenmore preferably less than about 2 ppm.

A preferred embodiment of the method comprises first diluting an aqueoussolution containing copper ions with water. Without limitation, apreferred dilution is about a five-fold volume dilution. The dilutedsolution is then treated with a sufficient amount of sulfuric acid toobtain a solution pH of from about 1.0 to less than about 2.5. Asufficient amount of iron powder is then added to the solution toprecipitate the copper ions as copper metal. The amount of iron to beadded to the solution may be determined empirically, or by measuring theoxidation-reduction potential of the solution using techniques known inthe art. The iron powder has a preferred mesh size of 230 or greater.The copper metal that precipitates is recovered using a filter press,and the solution thereafter has a copper ion concentration of less thanabout 5 ppm, preferably less than about 2 ppm. The recovered coppermetal has a purity of greater than about 60 percent.

An object of the present invention is to provide a process for treatingcircuit-board etching solutions with iron powder to precipitatedissolved copper ions as copper metal, thereby obtaining an etchingsolution that is substantially completely free of copper ions.

Still another object of the present invention is to provide a processwhereby a substantially pure copper metal can be obtained from usedcircuit-board etching solutions by treating such solutions with ironpowder wherein the copper ion concentration in the treated etchingsolution is decreased to about 2 ppm.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flow chart showing certain process steps used to practice aworking embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with one embodiment of the present invention,finely-divided metal powders are used as reducing agents to precipitatefrom aqueous solutions metals having a lower oxidation potential. Theprocess steps for one particular embodiment of the invention include:(1) obtaining an etching solution containing dissolved metal ions; (2)diluting the solution with water; (3) acidifying the diluted solutionwith an inorganic acid to a pH of below about 2.5, and preferably fromabout 1.0 to about 2.0; (4) adding a reducing metal, having a particlesize of 230 mesh or smaller, to the solution. Without limiting thepresent invention to one theory of operation, it appears that anoxidation/reduction reaction (redox reaction) occurs upon the additionof the reducing metal to the solution. For instance, iron may be used asa reducing metal if copper ions are dissolved in solution. Iron (0)reduces copper (II) to copper (0) metal; and (5) recovering theprecipitated metals.

Each of the reagents used to practice this invention, as well as each ofthe process steps summarized above, will be discussed in more detailbelow.

I. REAGENTS A. Circuit-Board Etching Solution

One embodiment of the present invention is particularly useful fortreating ammoniacal etching solutions to precipitate copper ions."Ammoniacal" refers to a solution containing ammonia or ammonium salts.Such solutions also may contain a number of other inorganic species,such as cupric chloride. Ammoniacal etching solutions are basic, havinga pH of typically greater than about 9.

A discussion of ammoniacal etching solutions is provided in Lee's U.S.Pat. No. 4,915,776, entitled "Process for Etching Copper with AmmoniacalEtchant Solution and Reconditioning the Used Etchant Solution." Lee'spatent is incorporated herein by reference. According to Lee's patent,the reactions between copper and the ammoniacal etching solution can besummarized as follows:

1. Reactions of Copper with Ammoniacal Etching Solutions REACTION 1

    Cu.sup.0 +Cu.sup.2+ (NH.sub.3)Cl.sub.2 →2Cu.sup.+ (NH.sub.3).sub.2 Cl

REACTION 2

    2Cu.sup.+ (NH.sub.3).sub.2 Cl+2NH.sub.4 OH+2NH.sub.4 Cl+1/2O.sub.2 →2Cu.sup.2+ (NH.sub.3).sub.4 Cl.sub.2 +3H.sub.2 O

The overall reaction can be determined by adding reactions 1 and 2, asshown below in Reaction 3.

REACTION 3

    Cu.sup.0 +2NH.sub.4 OH+2NH.sub.4 Cl+1/2O.sub.2 →Cu(NH.sub.3).sub.4 Cl.sub.2 +3H.sub.2 O

Reaction 3 shows that the overall reaction involves the reduction of Cumetal indicated as Cu⁰, i.e., copper in the zero oxidation state! withammonium hydroxide (NH₄ OH), ammonium chloride (NH₄ Cl) and oxygen. Theproduct is an oxidized ammonia copper complex with copper oxidized toCu²⁺.

An "oxidation reaction" refers to any reaction in which electrons aretransferred. "Oxidation" specifically refers to a chemical species whichloses electrons. "Reduction" refers to a chemical species that gainselectrons. The compound or element which gains electrons is referred toas the "oxidizing reagent"; the agent that loses electrons is referredto as the "reducing agent." By way of example, if copper (II) ions arepresent in an etching solution, then the oxidation reaction would referto the transfer of electrons from Fe (0) to copper (II). Iron loses twoelectrons and becomes positively charged, i.e., is oxidized. Copper (II)gains two electrons and becomes neutral, i.e., is reduced. Copper (II)is referred to as the "oxidizing agent."

The terms "reduction potential" and "oxidation potential" also are usedin this application. "Oxidation potential" refers to the potential of achemical species to lose or donate electrons. "Reduction potential"refers to the potential of a chemical species to gain electrons. Adiscussion of these concepts can be found in a number of standardchemistry texts, including, for example, Cotton and Wilkinson's"Advanced Inorganic Chemistry," published by John Wiley & Sons.

B. Metal Powder

One aspect of the present invention involves treating an etchingsolution with a finely-divided metal. "Finely divided" is defined belowin terms of mesh size. The metal is selected to have an oxidationpotential sufficient to reduce the metal ions in solution to the zerooxidation state. The selection of the metal will depend upon a number offactors, including the oxidation potentials of the metals in question.It will be appreciated by one skilled in the art that a number of metalshave a sufficient oxidation potential to reduce a particular ion, suchas copper (II). Whether a particular metal has a sufficient oxidationpotential can be determined either empirically, or by examining theelectrochemical series and calculating the free energy associated with aparticular reaction. This procedure is known to those of skill in theart. And, the electrochemical series is published in a number ofstandard references, including The Handbook of Chemistry and Physics,which is published by CRC Press.

By way of example, if the dissolved ion of interest is copper (II), thena number of metals potentially can be used to reduce the copper (II) tocopper (0). This can be determined by knowing the following potentialdata for copper (as shown in Table 1 below) and comparing such data tothe potential of the reducing agent:

                  TABLE 1                                                         ______________________________________                                        Cu.sup.+  + e.sup.-  = Cu.sup.0                                                                    E.sup.0  = 0.522 V                                       Cu.sup.2+  + e.sup.- E.sup.0  = 0.153 V                                       Cu.sup.2+  + 2e.sup.-  = Cu.sup.0                                                                  E.sup.0  = 0.340 V                                       ______________________________________                                    

By way of example and without limitation, metals that can be used toreduce copper (II) to copper (0) include: (1) magnesium (Mg²⁺ +2e⁻ =Mg;E⁰ =-2.375 V); (2) manganese (Mn²⁺ +2e⁻ =Mn; E⁰ =-1.029 V); and (3) iron(Fe²⁺ +2e⁻ =Fe; E⁰ =-0.409 V). For the present invention, applicant hasdetermined that iron is a preferred metal useful for reducing copper(II) ions in solution to copper (0). One reason for this, other than thefact that iron has a sufficient oxidation potential, is because iron isreadily available and relatively inexpensive.

As stated in the Background of the Invention, steel wool and ironparticles previously have been tried to precipitate copper ions fromsolution. These previous processes have proven unsatisfactory.Surprisingly, and contrary to the teachings of the prior art, applicanthas determined that a process involving using an iron powder that (1) isat least about 70 percent pure, and preferably at least about 90 percentpure, and (2) has a suitably small mesh size, produces an efficientreduction of copper ions to copper metal in acidic aqueous solutions.Currently, a mesh size of 230 or larger (i.e., smaller metal particles)is preferred. Particle sizes smaller than 230 mesh also are suitable forpracticing the invention. Working embodiments of the present inventionhave used metal powders having a mesh size of as small as about 400mesh. However, in terms of the percent copper ion remaining in thesupernatant following treatment with a reducing metal, such as iron, thepreferred mesh size appears to be 230 or greater. However, if thepercent copper metal obtained by the process is a guiding considerationin the selection of the metal particle size, then it currently appearsthat mesh sizes greater than about 300 and less than about 400 arepreferred. Thus, as used herein, the phrase "finely-divided metal"describes a metal having a sufficient purity and a sufficiently smallmesh size to efficiently carry out the desired reaction, namely thereduction of dissolved metals ions to the zero oxidation state. Theeffects of particle size on the reaction efficiency are consideredfurther in the examples provided below.

One skilled in the art will realize that iron powders meeting therequirements stated herein can be obtained from a number of sources. Onesource of an iron powder suitable for the present invention is HoeganaesCompany, of Riverton, N.J.

C. Inorganic Acid

Etching solutions, particularly ammoniacal etching solutions, are basicand therefore have a pH greater than about 7. More typically, ammoniacaletching solutions have a pH of greater than about 9. A surprisingincrease in reaction efficiency, in terms of the recovery of coppermetal and the decontamination of etching solutions, can be achieved bytreating etching solutions with finely-divided iron powders at a pH ofless than about 2.5, and even more preferably at a pH of from about 1.0to about 2.0.

Thus, the process of the present invention includes the step ofacidifying the solution prior to treating the solution with thefinely-divided metal powder. Acidification typically is accomplishedusing an inorganic acid. Suitable inorganic acids include, withoutlimitation, hydrochloric acid, nitric acid, sulfuric acid and phosphoricacid. A presently preferred inorganic acid is sulfuric acid. Sulfuricacid is readily available from a number of commercial sources, and isrelatively inexpensive. Furthermore, and again without limiting theinvention to one theory of operation, applicant currently believes thatthe intermediate formation of copper sulfate aids the subsequentreduction of aqueous copper (II) ion to copper (0) metal.

II. PROCESS STEPS

Using the reagents discussed above, the present invention provides aprocess whereby metal ions are reduced and precipitated from solution.Certain of the process steps of the invention will now be discussed inmore detail.

A. Dilution

A used industrial solution, such as ammoniacal etching solution,generally is diluted with water as a first process step. One purpose ofthis dilution is to decrease the heat generated by neutralizing thesolution upon the addition of an inorganic acid. For instance, the heatproduced by the exothermic neutralization reaction may be sufficient tomelt polymeric containers if the solution is not first diluted. Oneskilled in the art will realize that the amount of dilution required fora particular reaction will depend upon the amount of heat ofneutralization that is generated. To provide a guideline, the solutionshould be maintained at a temperature below about 100° C. so that thewater, and any compounds less volatile than water, is not vaporizedduring the process. Even more preferably, the dilution should besufficient so that the heat of neutralization does not increase thetemperature of the solution to much greater than about 50° C.

By way of example only, a five-fold volume dilution of ammoniacaletching solution sufficiently controls the temperature increase thatresults from the evolution of heat during addition of the inorganicacid. One of ordinary skill in the art will realize that there isnothing critical about a five-fold volume dilution. Smaller and largervolume dilutions also may suffice. Presently, it is believed that volumedilutions of from about a two-fold volume dilution to about a ten-foldvolume dilution, preferably from about a three-fold volume dilution toabout a six-fold volume dilution, and even more preferably about afive-fold volume dilution, will suffice for the present invention.

B. Acidification

Following dilution, the solution then is treated with an inorganic acid.The pH of the solution has a surprising influence on the efficiency ofthe reaction. The influence of pH on the reaction is considered furtherin the examples provided below. However, when a solution having a pH ofgreater than about 3.0 is treated with a finely-divided iron powder, therecovery of copper metal from the solution is unacceptably low.Specifically, when the pH of the solution is greater than about 3,atomic absorption spectrometry shows that the recovery of copper metalfrom the ammoniacal etching solution is less than about 20 percent.However, if the pH is less than about 3, then a specific pH need not beachieved in order to provide a satisfactory reaction. In other words,although pH is important, a range of pH values below about a pH of 3,and preferably about 2.5 or less, still provides satisfactory results.

C. Addition of Finely-Divided Metal Powder

Following the step of acidification, the solution is ready for treatmentwith a reducing metal. The finely-divided metal powder may be added tothe etching solution either batchwise or continuously. Currently, thecontinuous addition of metal powder is considered preferable tobatchwise addition. Presumably, continuous addition preventsagglomeration of the metal powders in solution, thereby providing moresurface area for reaction with the metal ions in solution.

The amount of metal powder added to the solution depends on the amountof oxidized metal ions in solution. If the total amount of dissolvedions in solution is known, or if the concentration of the metal ions insolution is known, then the amount of iron powder that should be addedto the solution also can be determined. For instance, if the metal insolution is copper, then about an equimolar amount of iron should beadded to the solution. This is because the reaction appears to involve a1:1 molar ratio of iron and copper. This can be seen by comparing thehalf reactions for copper and iron as shown below in Reaction 4:##EQU1## Reaction 4 shows that an equimolar amount of iron (0) should beadded to reduce the copper (II) ion to copper metal. However, inpractice it is generally preferred to add a slight molar excess of theiron powder to the copper-containing solution. Currently, from about a1.1 to about a 1.5 molar excess of iron is added to the solution toensure complete reduction of the copper (II) ions in solution to coppermetal.

An alternative method for determining the amount of reducing metal thatshould be added to the solution containing metal ions involvesdetermining the oxidation-reduction potential (ORP) of the solution. ORPinvolves measuring the voltage capacity of the solution. The method fordetermining the amount of iron powder to be added to a solutioncomprises first forming a solution having a known concentration of ionsin solution. For instance, if the solution is an ammoniacal solutioncontaining copper (II) ions, a standard solution can be formed havingabout the same concentration of copper (II) ions. The potential of thesolution is then measured using standard devices known in the art.Thereafter, iron powder is added to the solution until the supernatanthas a copper (II) ion concentration of about 2 ppm. The potential of thesolution is again measured. The value of the potential is then used as areference point for determining how much iron should be added to larger,industrial quantities of etching solution.

The reaction also is facilitated by agitating the reaction during, andperhaps subsequent to, the addition of the finely-divided iron powder.Agitation can be accomplished in a number of ways. For instance, theetching solution can be continuously stirred with devices known in theart, such as mechanical stirrers. It will be apparent that magneticstirrers are not preferred because if magnetic metals, such as ironparticles, are added to the solution such particles will be attracted tothe magnetic stir bar. Moreover, one skilled in the art also willunderstand that agitation methods other than mechanical stirring can beemployed. One example would be to sonically agitate the solution.

D. Recovering Reduced Metals

The finely-divided metal apparently reduces metal ions in the solutionthat have a lower oxidation potential than does iron. Once the etchantsolution is treated with a reducing metal, the dissolved metal ionshaving a lower oxidation potential are reduced and generally precipitatefrom solution. Sometimes the pH of the solution may require adjustmentupwards in order to cause the reduced metals to precipitate. Thecomposition containing the precipitated metal is then passed to a pressfilter to reduce the amount of water present in the precipitated solids.Although press filters are known in the art, a presently useful pressfilter is a J-press filter by JWI Inc., having a 630 mm plate size. Oncethe water content of the precipitated solids is reduced, the dehydratedsolids are recovered.

When the metal ions in solution are copper ions, the product from thepress filter is both copper metal and an etching solution that has hadthe copper ions substantially completely stripped therefrom. As usedherein, the phrase "substantially completely" means a solution that hasa metal-ion concentration of less than about 5 ppm, preferably less thanabout 3 ppm, and even more preferably from about 1 ppm to about 2 ppm.

The purity of the copper metal and the level of copper ion in thetreated etchant solution was determined using atomic absorption. For thepresent invention, a Perkin/Elmer Atomic Absorption Spectrometer, ModelNo. 2280, was used to determine both the purity of the copper metalobtained using the process and the level of copper ions still remainingin solution once the etching solution is treated according to thepresent invention.

An atomic absorption spectra of a copper sample obtained according tothe process of the present invention showed that a copper purity ofgreater than about 70 percent can be obtained. Perhaps more important,and in contrast to the teachings of the prior art, the concentration ofcopper ions in solution following treatment according to the presentinvention is less than about 3 ppm.

III. EXAMPLES

The following examples are provided for illustrative purposes only.These examples are in no way intended to limit the scope of theinvention to the particular aspects illustrated by such examples.

Example 1

This example describes treating a solution containing copper ion withiron powder without acidifying the solution prior to the addition ofiron powder. A 250 ml aliquot of an etchant was obtained having a copperconcentration of about 135 g/l. Thus, the 250 ml aliquot had a totalcopper content of about 33.75 grams, or about 0.53 moles. To thisaliquot was added 33.75 grams (0.60 mole) of iron having a mesh size ofabout 400. Thus, about a 1.12 molar excess of iron was added to thesolution. Upon the addition of iron, the solution remained unchanged inappearance, indicating that the amount of copper in solution remainedrelatively unchanged. Thus, treating a solution with iron powder withoutfirst acidifying the solution is an unacceptable method of recoveringcopper metal.

Example 2

This example considers what affect acidifying the solution has on theefficiency of the reaction. An etchant solution having the concentrationdiscussed in Example 1 was obtained. Sulfuric acid was added to thissolution. The pH target value was about 2.5. However, as the addition ofsulfuric acid continued, the viscosity of the solution slowly increased.At a pH of about 7.6, the solution turned unmanageably viscous. This maybe the result of the formation of copper hydroxide. This experiment wasdiscontinued when the solution no longer could be stirred.

Example 3

This example illustrates what affect diluting and acidifying thesolution has on the efficiency of the reaction. A 250 ml aliquot of anetching solution was obtained as described in Example 1. The aliquot wasthen diluted with water in a 1:5 volume ratio, i.e., a five-fold volumeexcess of water was added to the solution. The addition of sulfuric acidwas then begun. As the pH approached 7.5, the solution becameincreasingly turbid. The pH continued to decrease as more sulfuric acidwas added, and the solution remained turbid. At a pH of about 2.5 theturbidity disappeared and the solution became blue, apparentlyindicating the formation of aqueous copper sulfate. 33.75 grams of ironpowder, having a particle size of about 400 mesh, were then added to thesolution. An immediate reaction occurred, and metallic copper began toprecipitate. The solution was then filtered to remove the precipitatedcopper metal. An aliquot of the filtered supernatant was submitted foratomic absorption analysis. The results of the atomic absorption showedthat the copper concentration in the supernatant was about 2.67 ppm.

Example 4

This example further considers the affects of pH on the reactionefficiency. An etching sample was formed having a copper (II) ionconcentration of about 180 g/l copper. The etching first was dilutedwith a five-fold volume of water. The pH of the solution was thenadjusted to be about 3.2. A 250 ml aliquot of the etching solution(containing about 45 grams, 0.71 moles of copper) was then treated withabout 45 grams (0.805 moles; a 1.1 molar excess) of iron powder having amesh size of about 400. A reaction was observed and the solution turneda lighter shade of green. Copper metal also was seen to precipitate.However, the reaction did not continue to completion. It was clear thatacidifying the solution to a pH of only about 3.2 did not achieve thedesired result. As a result, acidifying to a pH of at least as low asabout 2.5 as stated above in Example 3 is preferred and results in amuch more efficient reduction of copper ion to copper metal.

Example 5

This example describes the treatment of a cupric chloride solution withiron powder according to the present invention. The solution was dilutedas described above in Example 4, and then sulfuric acid was added untila pH of about 2.5 was obtained. A slight excess (about 1.1 molar excess)of iron powder was then added to the solution. The solution turnedclear, with metallic copper particles settling to the bottom of thecontainer. Thereafter, the filtrate was analyzed by atomic absorptionspectrometry, which indicated that the filtrate contained about 100 ppmof copper.

Example 6

This example also evaluated what affect pH has on the reactionefficiency. The experiment was conducted exactly as in Example 5, exceptthat the initial pH was adjusted to about 1.5, instead of 2.5. Thefiltrate was then analyzed using atomic absorption spectrometry. Thecopper concentration of this sample was less than 1.0 ppm. Thus, theexamples discussed herein show that the pH should be less than about 3,preferably less than about 2.5, and even more preferably from about 1.5to about 2.5, in order to obtain a supernatant that is substantiallycompletely free of copper ion.

Example 7

The procedure of Example 5 was repeated with the addition of a final phadjustment step. This was done in order to obtain a solution having a pHacceptable for introduction into the sewer system. The final pH of thesolution was raised to 5.5 with the addition of 50 percent aqueoussodium hydroxide. The production of iron hydroxide was observed, inaddition to the metallic copper produced after the earlier processsteps. Thereafter, the filtrate was analyzed by atomic absorptionspectrometry, which indicated that the filtrate contained less than 1ppm copper. Additionally, the precipitated material was analyzed byatomic absorption spectrometry for metallic composition. This indicatedthat the composition of the product was approximately 50 percent copperand 50 percent iron.

Example 8

The procedure of Example 7 was repeated. However, a 50 percent aqueoussolution of magnesium hydroxide was used instead of sodium hydroxide forthe final pH adjustment step. A greatly lessened formation of ironhydroxide was observed, possibly due to the slower rate ofneutralization that occurs when using magnesium hydroxide. Theprecipitated material was then analyzed by atomic absorptionspectrometry for metallic composition. This analysis indicated that thecomposition of the product was greater than 70 percent copper, and lessthan 30 percent iron.

The following examples further explore the effects of the particle sizeof the reducing metal, such as iron, on the efficiency and productsproduced by the reaction. These examples demonstrate that particle sizesof about 230 mesh or greater (smaller particle sizes) produce superiorresults in terms of both the percent of copper metal obtained and theamount of copper ions remaining in the solution.

Example 9

A solution of CuSO₄.5H₂ O was prepared having a concentration of about100 g/L. The solution was acidified to a pH of about 1.5 by the additionof H₂ SO₄ (98 percent). A 100 milliliter aliquot of the acidifiedsolution was treated with 2.60 grams of iron powder (a 16 molar percentexcess) having a mesh size of less than 230. The solution was a bluecolor, indicating the presence of unprecipitated copper. The solutionwas neutralized using magnesium oxide, dried and weighed. The drymaterial was then dissolved in aqua regia to produce a solution suitablefor ICP analysis.

Example 10

A solution of CuSO₄.5H₂ O was prepared having a concentration of about100 g/L. The solution was acidified to a pH of about 1.5 by the additionof H₂ SO₄ (98 percent). A 100 milliliter aliquot of the acidifiedsolution was treated with 2.60 grams (a 16 molar percent excess) of 230mesh iron powder. The solution was a blue color, indicating the presenceof unprecipitated copper. The solution was neutralized using magnesiumoxide, dried and weighed. The dry material was then dissolved in aquaregia to produce a solution suitable for ICP analysis.

Example 11

A solution of CuSO₄.5H₂ O was prepared having a concentration of about100 g/L. The solution was acidified to a pH of about 1.5 by the additionof H₂ SO₄ (98 percent). A 100 milliliter aliquot of the acidifiedsolution was treated with 2.60 grams (a 16 molar percent excess) of 325mesh iron powder. The solution was a blue color, indicating the presenceof unprecipitated copper. The solution was neutralized using magnesiumoxide, dried and weighed. The dry material was then dissolved in aquaregia to produce a solution suitable for ICP analysis.

Example 12

A solution of CuSO₄.5H₂ O was prepared having a concentration of about100 g/L. The solution was acidified to a pH of about 1.5 by the additionof H₂ SO₄ (98 percent). A 100 milliliter aliquot of the acidifiedsolution was treated with 2.60 grams (a 16 molar percent excess) of 400mesh iron powder. The solution was a blue color, indicating the presenceof unprecipitated copper. The solution was neutralized using magnesiumoxide, dried and weighed. The dry material was then dissolved in aquaregia to produce a solution suitable for ICP analysis.

The results of Examples 9-12 are summarized below in Tables 2 and 3.

                  TABLE 2                                                         ______________________________________                                        % Copper and Iron of Dried Cake                                               Sample ID                                                                     Mesh Fe          % Cu   % Fe                                                  ______________________________________                                        >230             39.6   21.3                                                  230              32.4   25.0                                                  325              71.1   27.4                                                  400              54.0   19.6                                                  ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Contaminants in Supernatant Liquid                                            Sample ID                                                                     Mesh Fe        mg/L Cu  mg/L Fe                                               ______________________________________                                        >230           2.20     19.500                                                230            0.20     20.900                                                325            1.04     15.900                                                400            1.10     16.100                                                ______________________________________                                    

Table 2 clearly shows that when iron is used as the reducing metal, amesh size of about 325 is a preferred mesh size for increasing theamount of copper metal that is obtained in the dried cakes. Increasingthe amount of copper in the dried cakes is important because such metalcan then be reused. This does not mean that only metal powders having amesh size of 325 are suitable for the present invention. Rather, thedata presented in Table 2 illustrates that a range of metal particlesizes are suitable for practicing the invention, but that certainparticle sizes are superior to others in terms of the amount of copperthat can be recovered.

Table 3 illustrates the effects of particle size on the amount of copperion remaining in solution following treatment with iron powders. This isan important consideration in view of the environmental regulationsconcerning the disposal of copper-ion containing effluents. Table 3shows that an iron powder having a mesh size of about 230 reduced theamount of copper ions remaining in solution relative to iron powdershaving a larger mesh size (smaller particles). This is somewhatsurprising as one might have predicted that smaller metal particles,having an increased surface area, would provide better results. However,the results of Table 3 also demonstrate that all mesh sizes of 230 orgreater reduced the amount of copper ion remaining in solution to about1 ppm or less. Therefore, all such metal powders could be used topractice the present invention, which provides a method for reducing thecopper ions in solution to levels generally suitable for disposal inaccordance with current environmental regulations.

Having illustrated and described the principles of the present inventionin many preferred embodiments, it should be apparent to those skilled inthe art that the invention can be modified in arrangement and detailwithout departing from such principles. I claim all modifications comingwithin the spirit and scope of the following claims.

I claim:
 1. A method for recovering copper metal from an aqueoussolution containing copper ions, comprising:adding to the solution asufficient amount of an acid to obtain a solution pH of less than about2.5; adding iron particles to the solution in an amount sufficient toreduce the copper ions in solution, the iron particles having a meshsize of from about 230 to about 400; and adjusting the pH of thesolution to greater than about 5 by the addition of sufficient amountsof magnesium hydroxide, or compounds which form magnesium hydroxide inan aqueous system, to the solution.
 2. The method according to claim 1wherein the reducing metal is added to the solution continuously.
 3. Themethod according to claim 1 wherein the acid is an inorganic acid.
 4. Amethod for recovering copper metal from an aqueous solution containingcopper ions, comprising:treating the solution with a sufficient amountof an inorganic acid to obtain a pH of from about 1 to about 2.5; addingto the solution iron particles to reduce the copper ions in solution,thereby forming precipitated copper metal, the iron particles having amesh size of 230 or greater; adjusting the pH of the solution to about 5by the addition of sufficient amounts of a base; and recoveringprecipitated copper metal from the solution, the solution thereafterhaving a copper ion concentration of less than about 5 ppm.
 5. Themethod according to claim 4 wherein the iron has a mesh size of fromabout 230 to about
 400. 6. The method according to claim 4 wherein thethe solution has a copper ion concentration of less than about 2 ppmfollowing the step of recovering the copper metal.
 7. The methodaccording to claim 4 wherein the aqueous solution containing copper ionsis an ammoniacal circuit-board etching solution.
 8. The method accordingto claim 4 wherein the reducing metal is added to the solutioncontinuously.
 9. A method for recovering copper metal from a solutioncontaining copper ions, comprising:diluting the solution with at leastabout a two-fold volume excess of water; treating the solution with asufficient amount of an inorganic acid to obtain a solution pH of fromabout 1 to about 2.5; adding a sufficient amount of an iron powder tothe solution to precipitate the copper ions in solution as copper metal,the iron powder having a mesh size of 230 or greater; and recovering theprecipitated copper metal, the solution thereafter having a copper ionconcentration of less than about 5 ppm, the recovered copper metalhaving a purity of greater than about 60 percent.
 10. The methodaccording to claim 9 and further including the steps of:increasing thepH to greater than about 5.5 after the step of recovering the metal; anddischarging the solution into the environment.
 11. The method accordingto claim 9 and including the step of measuring changes in solutionpotential during the step of adding a sufficient amount of an ironpowder to the solution.
 12. A batch method for recovering copper metalfrom an ammoniacal circuit-board etching solution, comprising:dilutingthe etching solution with at least a two-fold volume excess of water;treating the solution with a sufficient amount of an acid to obtain asolution pH of from about 1 to about 2.5; adding an iron powder to thesolution to precipitate the copper ions in solution as copper metal, thesolution thereafter containing less than 2 ppm copper ions, the ironpowder having a mesh size of 230 or greater; increasing the pH of thesolution to at least 5 by the addition of magnesium hydroxide; andrecovering the precipitated copper metal.
 13. The method according toclaim 12 and including the step of continuously monitoring the potentialof the solution during the addition of the iron powder to the solution.